An auxiliary power unit (APU) system is often provided on an aircraft and is operable to provide auxiliary and/or emergency power to one or more aircraft loads. In conventional APU systems, a dedicated starter motor is activated during a starting sequence to bring a gas turbine engine up to self-sustaining speed. The gas turbine engine is then accelerated to operating speed. Once this condition is reached, a brushless, synchronous generator is excited and regulated so as to produce controlled electrical power at its terminals. The same start-up scheme is also applicable to start the main engines of the aircraft using the main engine starter/generator system.
As is known in the field, an electromagnetic machine may be operated as a motor to convert electrical power into motive power. Thus, in those applications where a source of motive power is required to start an engine, such as in an APU system or main engine starter/generator system, it is possible to omit the dedicated starter motor and operate the generator as a motor during the starting sequence to accelerate the engine to a self-sustaining speed. This capability is particularly advantageous in aircraft and electric car applications where size and weight must be held to a minimum.
The use of a starter/generator in starting and generating modes in an aircraft application has been realized by utilizing an inverter operating from a battery power source. The inverter provides control of a stator current vector coupled to the exciter machine with AC excitation to provide a main machine field flux when operated in the motoring mode. In a generating mode, conventional control of the exciter field is utilized to provide appropriate power quality. In such a system, a brushless three-phase synchronous generator operates in the generating mode to convert variable-speed motive power, supplied by a prime mover, into a fixed or variable-frequency AC power. The fixed or variable-frequency power is rectified and provided over a DC link to controllable static inverters or individual loads. The inverters are operated to produce constant-frequency AC power, which is then supplied over a load bus to one or more loads. The inverters can also be operated to produce variable voltage variable frequency AC voltage to supply various loads.
Torque produced at the shaft of the main machine is proportional to the main field flux in the main machine, and to the current in the main machine stator. To minimize the inverter KVA rating, it is desirable to maximize the main field flux in the main machine. Maximizing this flux requires that the excitation voltage applied to the exciter winding be increased to very high voltages. In applications where the maximum voltage is limited by potential insulation failure in windings, or connector voltage ratings, it is desirable to maximize the main field flux in the main field of the machine while at the same time minimizing the peak single phase excitation voltage applied to the exciter field winding.